Strength is your primary statistic. You should look for it
in all of your upgrades. It provides you with Attack Power, which
determines the damage your abilities do, and the healing you receive from
Death Strike. Strength also increases your chance to parry attacks.

Bonus Armor provides the best physical damage mitigation per stat
budget. Like Strength, it provides you with Attack Power, increasing damage
and healing from Death Strike.

Multistrike causes abilities to have an increased chance to hit/heal a second
and a third time, each time dealing 30% of the initial damage/heal.
In addition to that, Multistrikes from autoattacks also generate 15 Runic
Power.

Mastery is an important statistic for a Blood Death Knight.
You will want to get as much of it as possible, on all of your pieces of gear.
It provides you with larger shields from Mastery: Blood Shield, your
Mastery, which in turn translates into more absorbed damage and a smoother
damage intake.

Critical Strike causes abilities to deal 100% more damage, and all of your
Critical Strike chance is converted into Parry chance.

Versatility increases all your damage and healing done, and decreases
all damage taken.

Haste increases your attack speed and rune regeneration.

Most encounters have abilities that cannot be
mitigated or avoided. Also, you need to have a decent amount of health, in
order to give yourself the chance to be healed before a subsequent attack
kills you. As such, you need a minimum amount of Stamina. This amount
will, in most cases, simply come from the Stamina found innately on your gear.
Therefore, you should never have to go out of your way to gem for Stamina.

2.1. The Value of Multistrike

In all but the most survival-oriented stat priority, Multistrike has a high
value, despite the fact that it appears to offer very little. This is because
Death Knights are rather unique in that they rely much more on resources
(allowing them to use Death Strike more freely) than they do on raw
stats.